The detection of echo signals in CW systems where the echo return is deeply immersed in noise by the use of quadrature detection techniques is disclosed by Kalmus U.S. Pat. Nos. 3,432,855 and 3,733,581. In the basic embodiments therein shown, the outputs of quadrature mixer-detectors are correlated after one of the mixer outputs is subjected to a further 90.degree. phase shift. Since the signals encountered in an ultrasonic intrusion detector alarm system cover a range of about 3 octaves in the doppler detected signal, the additional phase shifter required by Kalmus must provide this range with relatively constant amplitude response. Phase shifters of this type at the relatively low frequencies encountered in an intrusion detection alarm system are expensive and difficult to provide. Furthermore, these prior art circuits require good balance in the two channels to maintain the signals substantially equal, and when they are multiplied or correlated or otherwise compared to cancel the noise components the desired target components are proportional to the square of the signal amplitudes since the two equal amplitude signals are multiplied together. In systems which do not employ signal correlation to produce an output proportional to the product of the two inputs some form of signal summing is employed and the requirements for channel balance to maintain the quadrature channel signals equal in amplitude presents a more severe requirement. Systems which operate by sampling the peak amplitude of one of the channels based on sampling pulses derived from the quadrature timing points available in the other channel discard a substantial portion of the useful information and substitute the sample values with the result that such systems can be readily jammed or desensitized with high frequency noise components which have symmetrical frequency spectrum when translated through the mixers. Such jamming can exceed the dynamic range of the system with such symmetrical signals and mask the statistically deficient sampled target information to prevent an alarm being given.